Corona reduction on printed circuit tuning stubs



M. J. WALLER Dec. 17, 1968 CORONA REDUCTION ON PRINTED CIRCUlT TUNING STUBS Filed Dec. 21. 1964 l I1. I lf NNW PATIENT,v AGENT 3,417,352 CORONA REDUCTION ON PRINTED CIRCUIT TUNING STUBS Milford J. Waller, Foxboro, Ontario, Canada, assignor to Northern Electric Company Limited, Montreal,

Canada Filed Dec. 21, 1964, Ser. No. 419,872

3 Claims. (Cl. 3153-84) ABSTRACT OF THE DISCLOSURE A strip transmission line filter in which quarter wavelength open-circuited stubs are modified from the normal rectangular configuration to prevent the occurrence of corona discharge at peak power of the order of 1 kw. caused by the high voltages at the open-circuited end of the resonant line. The modification consists of augmenting the area at the open-circuited end of the stub to form a substantially rounded or pear-shaped configuration, thereby altering the characteristic impedance of the stub and reducing the voltage at the open-circuited end.

This invention relates to microwave transmission elements of the strip line type and, in particular, to microwave strip line elements capable of handling large values of peak power without voltage breakdown occurring in the element, and to a method of producing this characteristic in strip line transmission elements.

Filters consisting of strip line elements have proved useful at microwave frequencies because they are compact and easily manufactured by conventional printed circuit techniques. One form of such filter consists of a conductive strip evenly spaced between two conductive plates. The two conductive plates are maintained at a common reference potential to form ground planes and the volume between the conductive strip and the plates is occupied by a suitable dielectric. The conductive strip has short stubs extending outwardly from it, also evenly spaced from the ground planes, to form the filter elements. By adjusting the spacing and length of these stubs the desired lter characteristic is obtained.

It has been found that, when filters having stubs approximately one quarter of a wavelength long are used to handle microwave peak powers of the order of 1 kilowatt (kw.) or above, they break down due to corona discharges occurring between a ground plane and the ends of stubs. These voltage breakdowns occur because such stubs in conjunction with the ground planes form open circuited quarter-wavelength lines on which the voltage is at a maximum at the open circuited end. When filters of this type are positioned between a transmitter and an antenna the peak power of the system is necessarily limited to a value less than that which will cause voltage breakdown in the filter.

It is, therefore, an object of this invention to provide an improved microwave lter of strip line configuration capable of handling large values of power.

It is a further object of this invention to provide a method of producing strip line elements which are capable of handling large values of peak power.

The filter of this invention departs from the normal practice in strip line filters in which the stub element attached to the conductive strip is of substantially rectangular shape. Instead the stub element is formed from two sections, the first being substantially rectangular in shape and attached to the conductive strip; the second section being of substantially rounded or pear-shaped configuration attached to the first section and forming the termination of the stub.

Further features and objects of this invention will beted States Patent O Mice 3,417,352 Patented Dec. 17, 1968 come apparent from the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a plan view of a printed circuit board forming the central conductor of a strip line filter embodying this invention,

FIGURE 2 is a cross-sectional view of the printed circuit board of FIG. 1 taken along the line 2-2,

FIGURE 3 is a side elevation, partially in cross-section, of a complete filter assembly embodying this invention, and

FIGURE 4 shows the configuration of a stub element in accordance with this invention.

Referring now to FIGURES 1 and 2 the central conductor 11 of a strip line filter is formed on one side of a conventional printed circuit board by etching away the copper plated on that side to leave the pattern shown in FIGURE 1 supported on insulating slab 13. The copper sheet 12 on the other side of insulating slab 13 is left in position.

The complete filter assembly, shown in FIGURE 3, includes a second printed circuit board having an insulating slab 13' and a copper sheet 12' on one side thereof. The insulating surface of slab 13 is arranged in engagement with central conductor 11 to form the assembly. A layer of dielectric compound may be used to encapsulate the edges of the central conductor. Copper sheets 12 and 12 form the ground planes of the filter with conductor 11, arranged symmetrically therebetween, forming the strip line.

The filter comprising insulating boards 13 and 13 with ground planes 12 and 12 and central conductor 11 is supported in position by heavy brass plates 21 and 22 which are in electrical contact with ground planes 12 and 12. Mechanical rigidity is provided by a large number of screws 28 and bolts 29 (of which only two are shown in FIGURE 3 for simplicity of drawing) which also serve the function of maintaining the ground planes at equal potential to prevent radiation from the filter. Conventional coaxial connectors 23 and 24 are attached to brass plates 21 and 22 by screws 27. The central conductors 25 and 26 of the coaxial connectors make contact with terminal portions 15 and 14, respectively, on the strip conductor 11 (FIGURE 1). Coaxial connector 36 has a control conductor 37 contacting the terminal portion 16 of strip conductor 11.

The particular configuration of strip conductor 11, shown in FIGURE 1, and which is used as an example, is designed to yield a diplex filter for use at frequencies of the order of 1000 mc./s. Terminal portion 16 is provided for connection to a transmission line adapted to carry transmitted energy in the frequency range 990-1040 mc./s. and received energy in the frequency range 1080- 1130 mc./s. Terminal portion 15 is provided for connection to a transmitting antenna and terminal portion 14 is provided for connection to a receiving antenna. A filter section, indicated generally at 17, lies between terminal portions 15 and 16. Another filter section, indicated generally at 18, lies between terminal portions 16 and 14. Section 17 comprises four stubs 7 and a terminating stub 6, and section 18 comprises four stubs 8 and a terminating stub 9.

Filter section 17 has low attenuation in the frequency band of the transmitted energy, 990-1040 mc./s., and couples energy in this frequency band from the transmission line to the transmitting antenna. Filter section 18 provides a high .attenuation between terminal portions 16 -and 14 in this frequency band and also provides the proper impedance match to the transmission line.

Filter section 18 has low attenuation in the frequency band of the received energy, 1080-1130 mc./s., and couples energy in this frequency band from the receiving antenna to the transmission line. Filter section 17 provides a high attenuation between terminal portions 15' and 16 in this frequency band and also provides the proper impedance match for the transmission line.

The stub sections such as 7 and 8 in FIGURE 1 are shown as having a configuration different from the normal rectangular shape. This modified shape of the stubs is shown in greater detail in FIGURE 4. In this stub the section or portion 31 of the stub, connected to the central conductor 11, is substantially rectangular and is similar to corresponding sections in prior art stubs. The outer or end section 3f) of the stub, which forms the open-circuited end is enlarged in area and given a smoothly curved periphery. This is achieved by providing outwardly tapering edges 32 terminating in a circular arc 33.

The modified shape of stub section 30 results in the characteristic impedance of that section being reduced. In consequence, the voltage developed between the opencircuited end of the stub and the ground planes is reduced, and the probability of voltage breakdown diminished. To achieve the same filter characteristics as would be obtained with normal rectangular stubs it is necessary to alter the length of the modified stub. In addition to reducing the voltage, the stu-b has a smoothly varying circular arc 33 which eliminates the discontinuities at the corners of the normal rectangular stub and facilitates the production of a smooth edge, free from sharp points, on the end of the stub. Any sharp points are obviously undesirable since the large value of electric field gradient produced Vat such points tends to initiate corona discharges.

The shape of the modified stub need not conform exactly to that shown in FIGURE 4. It is sufiicient that the area at the end of the stub be kept large and the boundary maintained as a smoothly varying curve. The increase in area at the end of the stub is limited to that value which can be compensated by a charge in length of the stub to restore the original filter characteristics.

Filters of the type described for operation in the frequency bands mentioned above have been constructed using both rectangular stubs and the novel shape of stub shown in FIGURE 4. It was found that both filters operated successfully at transmitter peak powers of 1.5 kw. but when the transmitter peak power was raised to 10 kw. the filter using rectangular stubs failed due to voltage breakdown occurring at the open-circuited end of the stubs. The filter using the modified shape of stub shown in FIGURE 4 operated successfully at transmitter peak powers of l kw.

Thus there has been disclosed a novel microwave strip line element capable of handling large values of microwave power without Voltage breakdown occurring.

I claim:

1. A microwave strip line filter comprising,

a pair of spaced apart ground plane members,

a central strip conductor intermediate said ground plane members and spaced therefrom,

a'stub element having first and second sections forming a quarter wavelength open-circuited resonant line with said ground planes,

said first section being substantially rectangular in shape having a first and second longer edge and a first and second shorter edge, said first shorter edge of said first section abutting upon said central conductor,

said second section having one straight edge abutting upon and coextensive With said second shorter edge of said first section,

said second section having a smoothly curved closed boundary extending from said straight edge and initially tapering outwards with respect to the direction of said first and second longer edges of said first section.

2. A microwave strip line filter comprising,

a pair of spaced apart ground plane members,

a central strip conductor intermediate said ground plane members and spaced therefrom,

a stub element having first, second and third sections forming an open-circuited resonant line with said ground planes,

said first section being a substantially rectangular conductor having a first and second narrower edge,

said first narrower edge of said first section abutting against said central conductor,

said second section being a substantially trapezoidal conductor having first and second parallel edges with said first edge being of a shorter length than said second edge,

said rst parallel edge of said second section abutting against and being coeXtensiVe with said second narrower edge of said first section,

said third section being a substantially semicircular conductor with the diameter abutting against and coextensive with said second parallel edge of said second section.

3. A microwave filter comprising,

a pair of spaced apart fiat parallel ground plane members,

a main conductor and stub conductor elements contiguous therewith and extending therefrom,

said main conductor and stub elements defining a plane and being positioned intermediate said pair of spaced apart ground plane members and separated therefrom by solid insulating material,

each said stub element having a first substantially rectangular section with a pair of shorter sides, a pair of longer sides and a longitudinally extending first axis and a second section of a generally bulbous configuration having a smoothly curved boundary symmetrically located with respect to a second axis and having a dimension at right angles to said second axis at the widest part thereof greater than the length of one of said shorter sides,

one of said shorter sides of said first section joining said main conductor and the other of said shorter sides joining said second section with the first and second axes in substantial alignment.

References Cited UNITED STATES PATENTS 2,401,296 4/ 1946 Fernsler.

2,937,347 5/ 1960 Matthei.

2,794,174 5/ 1957 Arditi et al. 2,964,718 12/1960 Packard.

2,751,558 6/1956 Grieg et al. 2,915,716 12/ 1959 Hattersley.

FOREIGN PATENTS 798,629 12/1959 Great Britain.

HERMAN SAALBACH, Primary Examiner. C. BARAFF, Assistant Examiner.

U.S. Cl. XR, 333-73 

